This invention relates to an improved coating for thermal emittance of an x-ray tube anode. In particular, the invention discloses a smooth coating which has improved high voltage stability on an x-ray tube anode as well as high thermal emittance.
As stated in U.S. Pat. No. 4,132,916 which is commonly assigned, it is well known that of the total energy involved in an electron beam striking an x-ray target, only 1% of the energy is converted into x-radiation with the remainder of about 99% being converted into heat. As explained in this patent under the "Background of the Invention", it is well known that the thermal emittance of x-ray tube anode targets can be enhanced to some extent by coating the target surface outside of the focal spot track with various coating compounds. The emitted heat is radiated through a glass envelope of the x-ray tube and ultimately to the oil circulating in the tube casing.
A variety of thermal emittance enhancing coatings have previously been used. For example, in the above referred to U.S. Pat. No. 4,132,916 there is described a coating composed of zirconium dioxide (ZrO.sub.2), hafnium oxide (HfO), magnesium oxide (MgO), strontium oxide (SrO) cerium dioxide (CeO.sub.2) and lanthanum oxide (La.sub.2 O.sub.3) or mixtures thereof stabilized with calcium oxide (CaO) or yttrium oxide (Y.sub.2 O.sub.3) and mixed with titanium dioxide (TiO.sub.2). This coating provides a "fused" coating on the x-ray anode. While this coating has been commercially acceptable, it has had some problems of low heat transfer, constrained by both low operating temperatures and low thermal emittance. In addition, there have been problems when applying the coating on some alloy substrates, and it has a tendency to "run" onto noncoated areas during the subsequent fusing treatment, thereby requiring further processing steps. More importantly, the process for applying it to the anode requires stringent parameters. One of the more serious problems has been that during vacuum firing of the coating on the anode, the temperature must be kept below 1400.degree. C. This also limits the user's ability to outgas the anodes prior to tube assembly.
In U.S. Pat. No. 4,029,828 there is described an x-ray tube target coating composed of 80-94% alumina (Al.sub.2 O.sub.3) and 6-20% TiO.sub.2. While this particular coating has good heat emissivity, there have been problems with its adhesion.
In U.S. Pat. No. 4,870,672 which is also commonly assigned, there is taught a thermal emittance coating composed of 40% to 70% by weight of TiO.sub.2, 20% to 40% by weight ZrO.sub.2 and 10% to 20% by weight Al.sub.2 O.sub.3. This particular coating is being used on a commercial product, but it has produced inconsistant fusing results.
In U.S. Pat. No. No. 4,090,103 there is disclosed a coating layer composed of molybdenum, tungsten, niobium and/or tantalum metals in combination with a 20-60 volume percent of a ceramic oxide such as TiO.sub.2, Al.sub.2 O.sub.3 and/or ZrO.sub.2. The in this and the '828 patent provide a "non-fused" coating on the x-ray anode which can present high voltage stability problems under normal operations.
In U.S. Pat. No. No. 4,953,190 which is also commonly assigned, there is described a combined fused and a non-fused anode coating which is composed of Al.sub.2 O.sub.3 present in an amount of 50% to 80% by weight and ZrO.sub.2 or La.sub.2 O.sub.3 and TiO.sub.2 present in an amount of 50% to 20% by weight with the TiO.sub.2 and ZrO.sub.2 or La.sub.2 O.sub.3 being present in a ratio in the range of 1:1 to 10:1.